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Re: Grouping {x,y z} data set

  • To: mathgroup at
  • Subject: [mg99615] Re: Grouping {x,y z} data set
  • From: Bill Rowe <readnews at>
  • Date: Sat, 9 May 2009 03:21:53 -0400 (EDT)

On 5/8/09 at 12:18 AM, graser at (graser) wrote:

>I have arbitrary data set.

>For example,

>Data={{x_1,y_1,z_1},{x_2,y_2,z_1},{x_1, y_2, z_3},........{x_n, y_m,

I assume this is *not* what you actually entered into
Mathematica. If it was, entering data literally as x_1 will
definitely be a problem

>I tried to group this data set depending on z value.

>I mean I try to select all x, y values with same z value.

In version 7, this is easily done using GatherBy. For example,

In[3]:= data =
   ArrayFlatten@{{RandomReal[1, {10, 2}],
      Transpose@{RandomInteger[1, {10}]}}};

In[4]:= GatherBy[data, Last]

Out[4]= {{{0.654913, 0.942989, 1}, {0.899263, 0.0620125,
    1}, {0.872745, 0.876903, 1}, {0.99796, 0.425091, 1}, {0.399351,
    0.169411, 1}, {0.435277, 0.683664, 1}}, {{0.857486, 0.913224,
    0}, {0.951074, 0.237923, 0}, {0.528556, 0.0531316, 0}, {0.796729,
    0.942358, 0}}}

In earlier versions of Mathematica this can be achieve using
Split after ordering the data appropriately


will group the data according to the last value in each row.
Note, while this will group the data in a manner equivalent to
GatherBy, the end result will typically have a different order.

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